The findings appear in the American Journal of Obstetrics and Gynaecology.

An episiotomy is a surgical procedure that should only be performed when it is absolutely necessary

Lead author Dr Edith Gurewitsch

When an infant's shoulders get stuck at the last moment of delivery, there is an urgent need to complete the delivery quickly because the baby can become starved of oxygen and suffer brain damage and suffocation.

Attempts to pull the baby from the birth canal can damage the child's shoulder.

One approach used to minimise damage and speed delivery is making a small surgical cut to the skin between a mother's vagina and rectum to widen the opening for delivery. This technique is called an episiotomy.

Manoeuvring

Current guidelines for UK doctors recommend episiotomy as one of the first steps in such situations. This gives the doctor more room in which to manoeuvre the 'stuck' baby.

But Dr Edith Gurewitsch and colleagues at Johns Hopkins do not believe this is necessary and say it does not make injury to the child less likely, and may actually increase the risk of shoulder trauma.

They looked at medical records of 127 cases where there was difficulty delivering the baby's shoulders.

It's a step forward in that it says you do not automatically have to do an episiotomy in this situation

Mr Patrick O'Brien, consultant obstetrician at University College Hospital in London

The rate of nerve damage to the babies' arms was the same regardless of whether the infants were delivered by a combination of physical manoeuvring and episiotomy or physical manoeuvring alone.

"An episiotomy is a surgical procedure that should only be performed when it is absolutely necessary," said Dr Gurewitsch.

"Episiotomy will only eliminate soft tissue barriers to delivery, whereas rotating the infant will realign its shoulders to fit within the mother's pelvis.

"It is the bony pelvis that is widely acknowledged as the main cause for the infant getting caught in the birth canal," she said.

Tearing

She said episiotomies put the mothers at increased risk of infection, bleeding and pelvic floor disorders, such as long-term discomfort during intercourse, flatulence and possible incontinence.

Patrick O'Brien, consultant obstetrician at University College Hospital in London, said: "It's very interesting and it would be great if this was proved to be the case because it would save women having unnecessary interventions.

"But they have not proved it."

He said only a randomised trial, where women were allocated to receive one of either delivery methods at random, would give a definitive answer.

However, this would be hard to do in real life because of the ethical problems such a study would create, he said.

"It's a step forward in that it says you do not automatically have to do an episiotomy in this situation," he said, and recommended guidelines be updated to take this into account.

But he pointed out that half of the women who did not have an episiotomy had actually torn.

"So it's not like you're protecting all of these women from the side effects the researchers describe, such as bleeding and infection."

He said, in some women, episiotomy might be the best option, preventing uncontrolled tearing.

A spokeswoman from the Royal College of Midwives said: "We welcome this piece of empirical work that challenges the traditional obstetric belief that an episiotomies must always be done for such cases."